This invention relates to cathode ray tube raster generator circuits in general and more specifically to a raster generator circuit independent of external clock requirements and having a self-adjusting line density.
Raster scanned cathode ray tubes (CRTs) generally require a synchronization signal to maintain a raster scan frequency synchronized with a modulated video signal input. In a number of special purpose applications, it is useful to vary the raster scan rate, thereby avoiding interference with other electronic circuits located in close proximity to the CRT, and to vary the amount of data represented on the display. In a typical apparatus, the X deflection signal is a sawtooth wave ramp with each tooth representing one side-to-side scan and the Y deflection input is likewise a sawtooth wave ramp having a lower frequency wherein the beam is swept repeatedly in the X axis, and the sweeping beam is gradually lowered or raised across the face of the display.
It is important that the raster sequence be started at the same time that the video modulating signal is input to the electron beam gun control to maintain an ascertainable video display. This typically requires an external clock, starting the X axis scan at the correct time relative to the Y deflection voltage.
One problem that has been noted is the varying efficiency of the electronics required in variable scan rate raster CRTs. The efficiency tends to be greater at relatively high scan rates and drops off as the scan rate is lowered. Additionally, when two or more CRTs are to be operated in an associated relationship, the requirement for an external clock requires an interconnect scheme and thus requires complex interconnection and coordination circuitry.